For road vehicles such as cars, it is known to provide a navigation system (such as GPS) which, along with stored map data, may give information regarding speed limits, intersections, traffic lights and so forth, so that the driver can be warned when it is determined that the vehicle may encounter a hazardous situation. It is also known to use “dead reckoning” to determine the current position of the vehicle relative to an earlier known position, if, for example, a GPS signal is lost (such a system may use accelerometers, gyroscopes and/or wheel rotation sensors to track the movements of the vehicle). It will be appreciated, however, that the further the vehicle progresses from the last known position, the degree of accuracy with which the position of the vehicle can be determined will steadily reduce.